Cellular base stations are well known in the art and typically include, among other things, baseband equipment, radios and antennas. FIG. 1 is a highly simplified, schematic diagram that illustrates a conventional cellular base station 10. As shown in FIG. 1, the cellular base station 10 includes an antenna tower 30 and an equipment enclosure 20 that is located at the base of the antenna tower 30. A plurality of baseband units 22 and radios 24 are located within the equipment enclosure 20. Each baseband unit 22 is connected to a respective one of the radios 24 and is also in communication with a backhaul communications system 44. Three sectorized antennas 32 (labelled antennas 32-1, 32-2, 32-3) are located at the top of the antenna tower 30. Three coaxial cables 34 (which are bundled together in FIG. 1 to appear as a single cable) connect the radios 24 to the respective antennas 32. In many cases the radios 24 are located at the top of the tower 30 instead of in the equipment enclosure 20 in order to reduce signal transmission losses.
Cellular base stations often use phased array antennas 32 that comprise a linear array of radiating elements. Typically, each radiating element is used to (1) transmit radio frequency (“RF”) signals that are received from a transmit port of an associated radio 24 and (2) receive RF signals from mobile users and pass these received signals to the receive port of the associated radio 24. Duplexers are typically used to connect the radio 24 to each respective radiating element of the antenna 32. A “duplexer” refers to a well-known type of three-port filter assembly that is used to connect both the transmit and receive ports of a radio 24 to an antenna 32 or to one or more radiating elements of multi-element antenna 32. Duplexers are used to isolate the RF transmission paths to the transmit and receive ports of the radio 24 from each other while allowing both RF transmission paths access to the radiating element(s) of the antenna 32.
FIG. 2 is a perspective view of a conventional duplexer 50. FIG. 3 is a perspective view of the conventional duplexer 50 of FIG. 2 with the cover plate removed therefrom. FIG. 4 is a perspective view of the duplexer 50 of FIGS. 2-3 with the top cover and resonators removed to more clearly show the cavities within the filter housing.
Referring to FIGS. 2-4, the conventional duplexer 50 includes a housing 60 that has a floor 62 and a plurality of sidewalls 64. An interior ledge 66 is formed around the periphery of the housing 60. Internal walls 68 extend upwardly from the floor 62 to divide the interior of the housing 60 into a plurality of cavities 70. Coupling windows 72 are formed within the walls 68, and these windows 72 as well as openings between the walls 68 allow communication between the cavities 70. Internally-threaded columns 74 and resonating elements 76 are provided within the housing 60. The resonating elements 76 may comprise, for example, dielectric resonators or coaxial metal resonators, and may be mounted onto selected ones of the internally threaded cavities 74. A cover plate 78 acts as a top cover for the duplexer 50. Screws 80 are used to tightly hold the cover plate 78 into place so that the cover plate 78 continuously contacts the interior ledge 66 and the top surfaces of the walls 68.
The duplexer 50 further includes an input port 82, an output port 84 and a common port 86. The input port 82 may be attached to an output port of a transmit path phase shifter (not shown) via a first cabling connection 83. The output port 84 may be attached to an input port of a receive path phase shifter via a second cabling connection 85. The common port 86 may connect the duplexer 50 to a radiating element of the antenna (not shown) via a third cabling connection (not shown). A plurality of tuning screws 90 are also provided. The tuning screws 90 may be adjusted to tune aspects of the frequency response of the duplexer 50 such as, for example, the center frequency of the notch in the filter response. It should be noted that the device of FIGS. 2-4 comprises two duplexers that share a common housing, which is why the device includes more than three ports (the device includes a total of six ports, although all of the ports are not visible in the views of FIGS. 2-4).
FIGS. 5A and 5B are perspective views of conventional tuning screws shown mounted in top covers of respective filters. Referring first to FIG. 5A, a tuning screw 100 is shown mounted in a top cover 120 of a filter housing. The top cover 120 has a plurality of apertures 130 extending therethrough (two apertures 130 are depicted in FIG. 5A, one of which has the tuning screw 100 inserted therein). A threaded nut 140 may be soldered above each aperture 130. Tuning screws 100 are threaded through the respective threaded nuts 140 to extend into the respective apertures 130 Only one tuning screw 100 is shown). The tuning screws 100 can readily be threaded further into and further out of the threaded nuts 140, and hence into and out of the cavity of the filter, and therefore may facilitate very precise tuning of the filter. While not shown, in other embodiments a thicker top cover 120 may be used that has threaded apertures formed therein which may eliminate the need for separate threaded nuts 140.
Referring to FIG. 5B, a cover 170 of a filter housing is depicted that includes a self-locking tuning screw 150 mounted therein. The self-locking tuning screw 150 is mounted in a threaded aperture 180 in the cover 170 (a second threaded aperture 180 is illustrated in FIG. 5B that does not have a tuning screw 150 therein). The self-locking tuning screw 150 may operate in the same fashion as the tuning screw 100 discussed above.